It is well known in the art to cast metallic workpieces from a melt of molten metal in a bath by either drawing the melt up into the mold assembly by vacuum, or to pressurize the air space above the melt, thereby forcing the melt up into the mold. These casting techniques have many inherent porblems, including low numbers of production, low yield due to impurities, and slow production counts. Expensive equipment is currently being used to alleviate these problems. Attempts have been made to first vacuum refine the molten metal to remove impurities which may be incorporated into the molded article thereby increasing the yield. In order to speed up production, methods have been devised for continuous casting and for providing series of vacuum casting molds conveyed in seriatim through vacuum chambers located above baths of molten metal within furnaces. These attempts have met with a certain degree of success, although there is a great deal of room for improvement.
In conventional low pressure die casting, a die is arranged above a melt holding pot while a tube extends from inside the die to a location below the level of the melt. The melt is forced up into the mold by pressurizing the air space above the molten metal bath pot, thereby forcing the molten metal up the tube into the die cavity. Such a method is described in U.S. Pat. No. 3,842,893 issued Oct. 22, 1974 to Booth.
In an attempt to increase the yield of acceptable molded articles, devices for vacuum refining of molten metal have been proposed. Present art devices for the vacuum refining of molten metal generally comprise a vacuum chamber containing a vessel of molten metal. Such devices suffer from a low yield because only a surface metal layer is subjected to the vacuum treatment. High hydrostatic molten metal pressure forms gas bubbles at a considerable depth within the metal, thereby causing thermodynamic infeasibility. Such a device is disclosed in U.S. Pat. No. 4,014,529 issued Mar. 27, 1977 to Puzhailo et al. The main object of the '529 patent is to provide a device for the vacuum refining of molten metal to enhance the degree of metal refining to substantially reduce harmful impurities.
As currently practiced, vacuum molds include ingates extending from their bottom, while being connected to a vacuum source at the top. The terminal end of the ingate is dipped into a molten metal bath while vacuum is applied, and the molten metal is drawn up into the mold cavity through the ingate. When molding thin walled articles, the vacuum casting technique produces an unusually low yield of acceptable product because the impurities on the surface of the molten metals, commonly known as dross of slag, are drawn up into the mold before the pure metal within the molten bath is drawn in. Because the molten metal is at an elevated temperature in the bath, the surface metal chemically reacts with the oxygen in the air above the bath to form metallic oxides and other undesirable impurities. Certain methods of molding utilize inert gas atmospheres over the molten metal bath to reduce the formation of dross or slag. This method, however, is costly and requires additional equipment and gas sources.
In conventional open atmosphere methods, the terminal end of the ingate is dipped through the dross layer on top of the molten metal during immersion and the dross is drawn into the mold cavity along with the pure metal by the vacuum and produces thin walled articles with dross and other impurities incorporated into the workpiece. For most molded articles, and especially for ones in the automotive field, the thin walled molded articles which include dross and other impurities are unacceptable and must be scrapped. Because it is critical for most of the molded articles to have high material integrity, this yields low numbers of manufactured products.
One attempt to increase the purity of molten metal being upcast is described in U.S. Pat. No. 4,211,270 issued Jul. 8, 1980 to Shinopulos et al. Described therein is cooled "open" mold assembly for the continuous, high speed casting of metallic strands, especially for upcasting strands of copper alloys. An "open" mold is one in which melt is continuously fed through the mold and immediately chilled at the end of the mold to form a product. Shinopulos et al. describes a cone assembly attached to the terminal end of the die extending from the mold which is made of a material non-contaminating to the melt being cast. The cone is attached to the bottom of the die portion, such that when the die portion of the mold assembly is inserted into the melt, the cone covers the end of the die portion, piercing the dross on the surface of the melt as it is immersed to reduce the quantity of foreign particles caught in the die during the upcasting process. The melt dissolves the cone and a starter rod bolt pushes a second cone off the die so that it floats to the side. This structure and method is particularly suited for continuous casting of metallic strands through "open" molds at high speeds, and is used for a single insertion of the die portion into the molten metal in the bath. Thereafter, the continuous casting of metallic strands is maintained until either: (1) the molten metal in the bath is depeleted; or (2) sufficient metallic strands have been made to meet production schedules. This configuration is not particularly well suited for a closed mold assembly because closed molds cannot inlcude the starter rod bolt used in the open mold assembly. Therefore, different structures are required for a closed mold assembly.
Therefore, it is a primary object of the present invention to provide a closed mold assembly including an expendable cap for molding single articles and cast parts.
It is another object of the present invention to provide a closed mold assembly suitable for vacuum casting thin wall molded articles having reduced impurities contained therein.
It is yet another object of the present invention to provide a closed mold assembly with upflow channels on the bottom of the mold assembly including channeled ribs for urging dross and other impurities in the top layer of the molten metal away from the ingate so that the impurities in the top layer of the melt are not incorporated into the molded article.